Process for oxidizing alkyl aromatic compounds

ABSTRACT

A process and a mixture for oxidizing an alkyl-aromatic compound comprises forming a mixture comprising the alkyl-aromatic compound, a solvent, a bromine source, a catalyst, and ammonium acetate; and contacting the mixture with an oxidizing agent at oxidizing conditions to produce an oxidation product comprising at least one of an aromatic aldehyde, an aromatic alcohol, an aromatic ketone, and an aromatic carboxylic acid. The solvent comprises a carboxylic acid having from 1 to 7 carbon atoms; and the catalyst comprises at least one of cobalt, titanium, manganese, chromium, copper, nickel, vanadium, iron, molybdenum, tin, cerium, and zirconium.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application No.61/360,281 which was filed on Jun. 30, 2010.

FIELD OF THE INVENTION

This invention relates to processes and mixtures useful for oxidizingalkyl aromatic compounds. More particularly, the invention relates tothe oxidation of alkyl aromatic compounds in the presence of a solvent,a catalyst, a bromine source, and ammonium acetate.

BACKGROUND OF THE INVENTION

The oxidation of alkyl aromatic compounds, e.g., toluene and xylenes areimportant commercial process. A variety of oxidation products may beobtained including aromatic carboxylic acids such as terephthalic acid(1,4-benzenedicarboxylic acid) and isophthalic acid(1,3-benzenedicarboxylic acid) which are used for example in the polymerindustry.

U.S. Pat. No. 2,833,816 discloses processes for oxidizing aromaticcompounds to the corresponding aromatic carboxylic acids. A process forthe liquid phase oxidation of alkyl aromatic compounds uses molecularoxygen, a metal or metal ions, and bromine or bromide ions in thepresence of an acid. The metals may include cobalt and/or manganese.Exemplary acids are lower aliphatic mono carboxylic acids containing 1to 8 carbon atoms, especially acetic acid.

U.S. Pat. No. 6,355,835 discloses a process for the preparation ofbenzene dicarboxylic acids by liquid phase oxidation of xylene isomersusing oxygen or air by oxidising in the presence of acetic acid assolvent, cobalt salt as catalyst and an initiator. The oxidation step isfollowed by flashing the said reaction mixture to remove volatilesubstances and cooling and filtering to get crude benzene di-carboxylicacid as a solid product and filtrate. Recrystallizing the crude benzenedi-carboxylic acid to obtain at least 99% purity and recycling of thefiltrate are also disclosed.

It is also known in the art that the oxidation products such as aromaticaldehydes, aromatic alcohols, aromatic ketones, and aromatic carboxylicacids may solidify or crystallize at oxidation conditions and/or as thereaction mixture cools. Thus, mixtures of oxidation products may beproduced which require further processing to increase the purity of thedesired product. For example, in the production of terephthalic acid,the oxidation product is often referred to as crude terephthalic acid asit contains impurities including color bodies and intermediate oxidationproducts especially 4-carboxybenzaldehyde (4-CBA). To obtain polymergrade or purified terephthalic acid, various purification steps areknown in the art including: washing the crude terephthalic acid withwater and/or a solvent, additional oxidation or crystallization steps,and reacting a solution of dissolved crude terephthalic acid withhydrogen at hydrogenation conditions usually including a catalystcomprising palladium and carbon. Often several purification steps areused.

U.S. Pat. No. 7,692,036 discloses an optimized process and apparatus formore efficiently and economically carrying out the liquid-phaseoxidation of an oxidizable compound. Such liquid-phase oxidation iscarried out in a bubble column reactor that provides for a highlyefficient reaction at relatively low temperatures. When the oxidizedcompound is para-xylene and the product from the oxidation reaction iscrude terephthalic acid (CTA), such CTA product can be purified andseparated by more economical techniques than could be employed if theCTA were formed by a conventional high-temperature oxidation process.

There remains a need in the art for alternate processes that enable theoxidation of alkyl aromatic compounds including oxidation processes thatproduce aromatic carboxylic acids. Oxidation processes that producehigher purity oxidation products are desirable to eliminate or minimizepurification costs. Resulting products with different ratios ofcontaminant may provide new intermediates useful as raw materials inother applications.

SUMMARY OF THE INVENTION

It has been discovered that the oxidation of an alkyl-aromatic compoundin the presence of ammonium acetate may provide a solid oxidationproduct having a different composition relative to that observed inconventional processes. In an embodiment, the invention provides a solidoxidation product of higher purity.

In an embodiment, the invention is a process for oxidizing analkyl-aromatic compound comprising: forming a mixture comprising thealkyl-aromatic compound, a solvent, a bromine source, a catalyst, andammonium acetate; and contacting the mixture with an oxidizing agent atoxidizing conditions to produce a solid oxidation product comprising atleast one of an aromatic aldehyde, an aromatic alcohol, an aromaticketone, and an aromatic carboxylic acid; wherein the solvent comprises acarboxylic acid having from 1 to 7 carbon atoms, and the catalystcomprises at least one of cobalt, titanium, manganese, chromium, copper,nickel, vanadium, iron, molybdenum, tin, cerium, and zirconium.

In another embodiment, the invention is a mixture for oxidizing analkyl-aromatic compound comprising: the alkyl-aromatic compound, asolvent, a bromine source, a catalyst, and ammonium acetate; wherein thesolvent comprises a carboxylic acid having from 1 to 7 carbon atoms, andthe catalyst comprises at least one of cobalt, titanium, manganese,chromium, copper, nickel, vanadium, iron, molybdenum, tin, cerium, andzirconium.

DETAILED DESCRIPTION OF THE INVENTION

In general, the invention relates to processes and mixtures orcompositions for the oxidation of alkyl aromatic compounds to produceone or more oxidation products, such as, aromatic aldehydes, aromaticalcohols, aromatic ketones, and aromatic carboxylic acids. Exemplaryprocesses according to the invention include the oxidation ofpara-xylene to terephthalic acid, the oxidation of ortho-xylene tophthalic acid, and the oxidation of meta-xylene to isophthalic acid.

Suitable alkyl aromatic compounds or feeds to be oxidized includearomatic compounds comprising at least one benzene ring having at leastone alkyl group. Methyl, ethyl, and isopropyl alkyl groups are preferredalkyl groups but are not required. In an embodiment, the alkyl aromaticcompound is selected from the group consisting of toluene, para-xylene,ortho-xylene, and meta-xylene. The feed may comprise more than one alkylaromatic compound. As the oxidation reaction generally proceeds throughsuccessive degrees of oxidization, suitable feed compounds also includepartially oxidized intermediates relative to the desired oxidizedproduct. For example, in the production of terephthalic acid, the alkylaromatic feed may comprise para-toluic acid and/or 4-carboxybenzaldehyde(4-CBA).

In an embodiment the invention is mixture or composition comprising thealkyl-aromatic compound, a solvent, a bromine source, a catalyst, andammonium acetate. The solvent comprises a carboxylic acid having from 1to 7 carbon atoms. In an embodiment, the carboxylic acid comprisesacetic acid. The solvent may contain more than one carboxylic acid. Forexample the solvent may further comprise benzoic acid. In anotherembodiment, the carboxylic acid of the solvent is acetic acid.

Optionally, the solvent may further comprise water. The water may beadded to the mixture or generated in the mixture during the oxidationprocess. In an embodiment, the amount of water ranges from about 0.01 wt% to about 5 wt %, relative to the weight of the carboxylic acid havingfrom 1 to 7 carbon atoms. The amount of water may range from about 0.1wt % to about 2 wt %, relative to the weight of the carboxylic acidhaving from 1 to 7 carbon atoms. In an embodiment, the ratio of solventto alkyl aromatic compound in the mixture ranges from about 1.5:1 toabout 6:1 by weight. The ratio of solvent to alkyl aromatic compound mayrange from about 2:1 to about 4:1 by weight.

The catalyst comprises at least one of cobalt, manganese, titanium,chromium, copper, nickel, vanadium, iron, molybdenum, tin, cerium andzirconium. In an embodiment, the catalyst comprises cobalt andmanganese. The metal may be in the form of an inorganic or organic salt.For example, the metal catalyst may be in the form of a carboxylic acidsalt, such as, a metal acetate and hydrates thereof. Exemplary catalystsinclude cobalt (II) acetate tetrahydrate and manganese (II) acetate,individually or in combination. In an embodiment, the amount ofmanganese (II) acetate is less than the amount of cobalt (II) acetatetetrahydrate by weight.

The amount of catalyst used in the invention may vary widely. Forexample, the amount of cobalt may range from about 0.001 wt % to about 2wt % relative to the weight of the solvent. In an embodiment, the amountof cobalt ranges from about 0.05 wt % to about 2 wt % relative to theweight of the solvent. The amount of manganese may range from about0.001 wt % to about 2 wt % relative to the weight of the solvent. In anembodiment, the amount of manganese ranges from about 0.05 wt % to about2 wt % relative to the weight of the solvent. In another embodiment, theratio of cobalt to manganese ranges from about 3:1 to about 1:2 byweight on an elemental metal basis.

Bromine sources are generally recognized in the art as being catalystpromoters and include bromine, ionic bromine, e.g. HBr, NaBr, KBr,NH₄Br; and/or organic bromides which are known to provide bromide ionsat the oxidation conditions, such as, benzylbromide, mono- anddi-bromoacetic acid, bromoacetyl bromide, tetrabromoethane, ethylenedi-bromide. In an embodiment, the bromine source comprises or consistsessentially of or consists of hydrogen bromide. The amount of hydrogenbromide may range from about 0.01 wt % to about 5 wt %, relative to theweight of the solvent. In another embodiment, the amount of hydrogenbromide ranges from about 0.05 wt % to about 2 wt %, relative to theweight of the solvent.

The mixture also includes ammonium acetate. In an embodiment, the amountof ammonium acetate ranges from about 1 wt % to about 100 wt %, relativeto the weight of the solvent. Thus, in a broad embodiment, the inventionis a mixture comprising an alkyl-aromatic compound, a solvent, a brominesource, a catalyst, and ammonium acetate wherein the solvent comprises acarboxylic acid having from 1 to 7 carbon atoms and optionally water,and the catalyst comprises at least one of cobalt, titanium, manganese,chromium, copper, nickel, vanadium, iron, molybdenum, tin, cerium, andzirconium. In an embodiment, the mixture comprises an alkyl-aromaticcompound, a solvent comprising acetic acid and optionally water, abromine source comprising hydrogen bromide, a catalyst comprising cobaltand manganese, ammonium acetate.

In another embodiment, the invention is a process for oxidizing analkyl-aromatic compound comprising: forming a mixture comprising thealkyl-aromatic compound, a solvent, a bromine source, a catalyst, andammonium acetate; and contacting the mixture with an oxidizing agent atoxidizing conditions to produce an oxidation product comprising at leastone of an aromatic aldehyde, an aromatic alcohol, an aromatic ketone,and an aromatic carboxylic acid. The solvent comprises a carboxylic acidhaving from 1 to 7 carbon atoms and optionally water, and the catalystcomprises at least one of cobalt, titanium, manganese, chromium, copper,nickel, vanadium, iron, molybdenum, tin, cerium, and zirconium. Themixture of the process may comprise an alkyl-aromatic compound, ammoniumacetate, a solvent comprising acetic acid and optionally water, abromine source comprising hydrogen bromide, a catalyst comprising cobaltand manganese.

Oxidation processes according to the invention may be practiced inlaboratory scale experiments through full scale commercial operations.The process may be operated in batch, continuous, or semi-continuousmode. The mixture described above may be formed in various ways. Theorder of addition of the mixture components (e.g. alkyl-aromaticcompound, solvent, bromine source, catalyst, and ammonium acetate) isnot critical. In an embodiment, two or more components may be combinedor mixed before being combined or mixed with other components. At leasta portion of the mixture provides a liquid phase, though dissolution ofone or more of the mixture components may not be complete at any or sometime during the process. The liquid phase may be formed by mixing thecomponents at ambient conditions. In another embodiment, the liquidphase is formed as the temperature of the mixture increases to theoxidation temperature. The mixture may be formed prior to the oxidationstep, in the same or different vessel as that used in the oxidationstep. In another embodiment, the mixture is formed in an oxidationreactor, e.g. adding various streams of the components individuallyand/or in combination to a continuous or semi-continuous oxidationreactor. The mixture, and/or various streams of the mixture componentsmay be heated before they are mixed together.

Though many conventional alkyl aromatic oxidation processes aretypically conducted in a mixed phase, and often include three phases(e.g. solid, gas, and liquid), they are frequently referred to in theart as “liquid phase” oxidation processes because the oxidationconditions are maintained to provide at least a portion of the mixturein the liquid phase. It is also known in the art that the number ofphases present may vary over time during the process. Processesaccording to the instant invention may also be conducted in a liquidphase or mixed phase in a similar manner as known in the art.

Conventional, liquid phase oxidation reactors as known in the art may beused to practice the invention. Examples include vessels, which may haveone or more mechanical agitators, and various bubble column reactorssuch as those described in U.S. Pat. No. 7,692,036. It is also known todesign, operate, and control such reactors and the oxidation reactionfor the oxidation conditions employed including, e.g., the temperature,pressure, liquid and gas volumes, and corrosive nature of the liquid andgas phases where applicable. See, e.g. U.S. Pat. No. 7,692,036 and U.S.Pat. No. 6,137,001.

The process of the invention also comprises at least one step ofcontacting the mixture with an oxidizing agent at oxidizing conditionsto produce an oxidation product comprising at least one of an aromaticaldehyde, an aromatic alcohol, an aromatic ketone, and an aromaticcarboxylic acid. Thus, in another embodiment, the mixture furthercomprises an oxidizing agent. In yet another embodiment, the mixturefurther comprises an oxidation product comprising at least one of anaromatic aldehyde, an aromatic alcohol, an aromatic ketone, and anaromatic carboxylic acid. The oxidation product may comprise an aromaticcarboxylic acid.

Suitable oxidizing agents for the process provide a source of oxygenatoms to oxidize the alkyl aromatic compound and/or an intermediateoxidization product at the oxidation conditions employed. Examples ofoxidizing agents include peroxides, superoxides, and nitrogen compoundscontaining oxygen such as nitric acids. In an embodiment, the oxidizingagent is a gas comprising oxygen, e.g. air, carbon dioxide, andmolecular oxygen. The gas may be a mixture of gasses. The amount ofoxygen used in the process is preferably in excess of the stoichiometricamount required for the desired oxidation process. In an embodiment, theamount of oxygen contacted with the mixture ranges from about 1.2 timesthe stoichiometric amount to about 100 times the stoichiometric amount.Optionally, the amount of oxygen contacted with the mixture may rangefrom about 2 times the stoichiometric amount to about 30 times thestoichiometric amount.

Oxidizing conditions generally include a temperature ranging from about125° C. to about 275° C. and a pressure ranging from about atmospheric,i.e. 0 MPa(g), to about 6 MPa(g) and a residence time ranging from about5 seconds to about 2 weeks. That is, the mixture has a temperature and apressure within these ranges and may be maintained within these rangesfor a period of time within the residence time range. In anotherembodiment, the temperature ranges from about 175° C. to about 225° C.;and the temperature may range from about 190° C. to about 235° C. In anembodiment, the pressure ranges from about 1.2 MPa(g) to about 6.0MPa(g); and the pressure may range from about 1.5 MPa(g) to about 6.0MPa(g). In a further embodiment, the residence time ranges from about 10minutes to about 12 hours. The oxidation temperature, pressure andresidence time may vary based on a variety of factors including forexample, the reactor configuration, size, and whether the process is,batch, continuous, or semi-continuous. An oxidation condition may alsovary based on other oxidation conditions. For example, use of aparticular temperature range may enable use of a different residencetime range.

In an embodiment, the oxidation product produced by the instantinvention may precipitate, crystallize, or solidify in a liquid phasemixture at the oxidation conditions and/or as the mixture cools. Thus, amixture according to the invention may further comprise a solidoxidation product. Other compounds, including color bodies, and otheroxidation products may solidify with or be trapped in the solidoxidation product thus reducing the purity of the desired product. In anembodiment, the mixture comprises a liquid phase. The mixture maycomprise a gas phase such as when the oxidizing agent is added as a gas.The mixture may comprise a solid phase e.g. a mixture component, anoxidation product, or a by-product fails to dissolve or solidifies inthe mixture. In an embodiment, the mixture comprises a liquid phase, asolid phase and optionally a gas phase. In another embodiment, themixture comprises a liquid phase and a gas phase.

As noted above and discussed below, it has been discovered that theinvention may be used to produce a solid oxidation product having adifferent composition relative to those observed in conventionalprocesses. In addition, the invention provides new ways to control thelevel of various contaminants in the solid oxidation product. In anembodiment, a process according to the invention further comprisesforming the oxidation product as a solid, optionally at the oxidizingconditions, to produce the solid oxidation product and a mother liquor.The solid oxidation product may form as the mixture cools. The solidoxidation product may be separated from the mother liquor, i.e. liquidphase, and the mother liquor of the process may be recycled and reusedin the contacting step or other steps of the process described below.

Processes according to the invention, may comprise one or moreadditional oxidizing steps. In an embodiment a second oxidation stepincludes a second oxidizing temperature that is lower than thetemperature of the first oxidizing step. Processes according to theinvention may include additional contacting steps of the invention asdescribed herein, and/or the invention may be combined with otheroxidizing steps such as conventional oxidizing steps known in the art.Multiple contacting or oxidation steps may be conducted in series and/orparallel and may be combined with other process steps such aspurification steps described herein.

In a sequential embodiment, the invention includes a second oxidationstep wherein a portion or all of the solid oxidation product, or themother liquor, or both the solid oxidation product and the mother liquorproduced in the first oxidation step forms a second mixture with asecond solvent, a second bromine source, ammonium acetate, and a secondcatalyst. The second mixture is contacted with a second oxidizing agentat second oxidizing conditions to produce a second solid oxidationproduct comprising at least one of an aromatic aldehyde, an aromaticalcohol, an aromatic ketone, and an aromatic carboxylic acid. The secondsolvent comprises a carboxylic acid having from 1 to 7 carbon atoms, andthe catalyst comprises at least one of cobalt, titanium, manganese,chromium, copper, nickel, vanadium, iron, molybdenum, tin, cerium, andzirconium. The second solvent, second bromine source, second catalyst,and second oxidation conditions may individually or collectively be thesame or different from those of the first oxidation step. Optionally, aportion of the alkyl aromatic compound may be included in the secondmixture. The optional elements and optional steps described for thefirst oxidation step above are equally applicable to this secondoxidation step.

In a parallel embodiment, the invention further comprises a secondoxidation step wherein a second mixture comprising a portion of thealkyl-aromatic compound, a second solvent, a second bromine source, anda second catalyst is formed. The second mixture is contacted with asecond oxidizing agent at second oxidizing conditions to produce asecond solid oxidation product comprising at least one of an aromaticaldehyde, an aromatic alcohol, an aromatic ketone, and an aromaticcarboxylic acid. The second solvent comprises a carboxylic acid havingfrom 1 to 7 carbon atoms and the second catalyst comprises at least oneof cobalt, titanium, manganese, chromium, copper, nickel, vanadium,iron, molybdenum, tin, cerium, and zirconium. Optionally, the secondmixture further comprises ammonium acetate. The second solvent, secondbromine source, second catalyst, and second oxidation conditions mayindividually or collectively be the same or different from those of thefirst oxidation step. The optional elements and optional steps describedfor the first oxidation step above are equally applicable to this secondoxidation step

In another embodiment, the invention further comprises purifying thesolid oxidation product. Purifying may comprise one or more additionalsteps to isolate and purify the solid oxidation product. Examples ofpurifying steps include: separating wherein the solid oxidation productis separated from the mother liquor or another liquid phase such as byfiltration and/or centrifugation; washing wherein the solid oxidationproduct is washed, for example with water and/or another solventcomponent; drying the solid oxidation product; and hydrogenationprocesses. Such additional processing steps have been described in thegeneral literature and are well known to those of ordinary skill in theart to be used in various combinations to purify solid oxidationproducts of the invention. See for example, the references cited in thisapplication and the art cited therein.

A purification step of instant invention may further comprise a one ormore solvent contacting steps. A solvent contacting step comprisescontacting a solid oxidation product, also including washed or driedsolid oxidation products, with a second solvent comprising at least oneof water, a carboxylic acid having from 1 to 7 carbon atoms, and amother liquor to produce a second solid oxidation product. In anembodiment, the second solvent is selected from the group consisting ofthe mother liquor, a carboxylic acid having from 1 to 7 carbon atoms,water, and combinations thereof. Solvent contacting may leach impuritiesfrom the solid oxidation product, and/or the solid oxidation product maybe partially or completely dissolved in the solvent. Solvent contactingconditions include a solvent contacting temperature. The solventcontacting temperature may be lower than the oxidation temperature. Inan embodiment, the solvent contacting temperature is at least 20° C.lower than the oxidation temperature. Solvent contacting may bepracticed for example in the one or more crystallizers that follow theoxidation reactor in some conventional processes. The second solidoxidation product may solidify, precipitate, or crystallize in thesecond solvent of the solvent contacting step.

In an embodiment, the invention is a process further comprisingcontacting the solid oxidation product and a solution comprising themother liquor at contacting conditions including a second temperature toproduce a second solid oxidation product and a second mother liquor; thesecond temperature being lower than the oxidation temperature.Optionally the process further comprises separating the second solidoxidation product from the second mother liquor; and the process mayfurther comprise purifying the second solid oxidation product.

The solid oxidation product made by the instant invention may bepurified by known methods including the use of a hydrogenation step. Inan exemplary embodiment, a hydrogenation step is not required. In anembodiment, a process according to the invention includes one or morepurification steps that exclude hydrogenation steps. That is, thepurifying process steps may be selected from the group of process stepsconsisting of washing, separating, drying, solvent contacting, andcombinations thereof.

EXAMPLES

The examples are presented to further illustrate some aspects andbenefits of the invention and are not to be considered as limiting thescope of the invention.

Example 1

Experimental procedure: In a fume hood, load a Parr reactor with thespecified amounts of components for the given experiment seal thereactor. The Parr reactor includes a gas distributor to disperse the gasthrough a 1.6 mm opening into the liquid, a mechanical gas entrainmentstirrer, and baffles to ensure thorough mixing. Install the Parr reactorin a heater assembly at room temperature and connect a gas supply lineto the reactor and a condenser to the reactor outlet. During operation,gases exit the reactor through the condenser then a trap, then aback-pressure regulator. Connect a safety vent having a rupture disk,and thermocouples to the reactor. Connect a cooling water recirculatorto the condenser and begin to recirculate cooling water. Pressure testthe Parr reactor at room temperature and 1.4 MPa(g) (200 psig) usingnitrogen until there is no decrease in pressure for 15 minutes. Set theback pressure regulator on the reactor outlet to the experimentalpressure and pressure test the reactor under nitrogen. Begin raising thereactor temperature to the experimental temperature under the nitrogenatmosphere. Always follow all instructions for the specific reactorincluding temperature and pressure limits. When the reactor reaches thedesired temperature begin adding air at the experimental rate andmonitor the reactor temperature and pressure for the duration of thetest. During the test, the air flow into the reactor is maintained at1250 standard cm³ per minute, the pressure is maintained at 4.1 MPa(g),and the stirrer is maintained at 1600 rpm. At the end of the test shutoff the heater, cut the air flow and allow the reactor to cool. When thereactor cools to less than about 35° C., open the back pressure valve,stop the cooling water, and remove and empty the reactor to obtain thesolid oxidation product and mother liquor.

The mother liquor and products are filtered under vacuum to separate thesolids and liquid. The solids are then mixed with approximately 100 ccdeionized water at room temperature and decanted. The room temperaturedeionized water mixing and decanting is repeated two additional times. Afourth wash with deionized water is heated to approximately 95° C. for30 minutes and then filtered. The solids are dried at 80° C. for 8-24hours before analyzing.

Examples 2-3

Examples 2-3 were individual tests conducted using the equipment andprocedure given in Example 1. The components of the mixture, given ingrams, operating temperature and time, and results are given in Table 1.

TABLE 1 Example Number 2 3 Oxidation Temperature, ° C. 200 200 OxidationTime, hours 6 6 Mixture Components, grams para-xylene 20 20 glacialacetic acid 100 80 water 2 0.4 ammonium acetate 0 20 hydrogen bromide0.4 0.4 cobalt (II) acetate tetrahydrate 0.8 0.8 manganese (II) acetate0.6 0.6 Analysis of solid product terephthalic acid, wt % 98.6 99.54-carboxybenzaldehyde, wt % 1.10 0.37 para-toluic acid, wt % 0.26 0.07benzoic acid, ppm-wt 230 0 4-hydoxymethylbenzoic acid, ppm-wt 555 83

Example 2 (Comparative)

Conventional test run without ammonium acetate to demonstrate the levelof impurities made using conventional solvents under standard oxidizingconditions.

Example 3

Same oxidizing conditions as Example 2 except ammonium acetate wassubstituted for some of the acetic acid. Incorporating ammonium acetatesignificantly increased the purity of the terephthalic acid and reducedthe concentrations of 4-CBA, p-toluic acid, benzoic acid, and4-hydoxymethylbenzoic acid.

The invention claimed is:
 1. A mixture for oxidizing an alkyl-aromaticcompound comprising: the alkyl-aromatic compound, a solvent, a brominesource, a catalyst, and ammonium acetate, wherein the amount of ammoniumacetate ranges from about 1 wt % to about 100 wt %, relative to theweight of the solvent; wherein the solvent comprises a carboxylic acidhaving from 1 to 7 carbon atoms, and optionally water, and the catalystcomprises at least one of cobalt, titanium, manganese, chromium, copper,nickel, vanadium, iron, molybdenum, tin, cerium, and zirconium.
 2. Themixture of claim 1 further comprising an oxidizing agent.
 3. The mixtureof claim 1 wherein the carboxylic acid comprises acetic acid.
 4. Themixture of claim 1 wherein the alkyl-aromatic compound is selected fromthe group consisting of toluene, para-xylene, ortho-xlyene, andmeta-xylene.
 5. The mixture of claim 1 wherein the catalyst comprisescobalt and manganese, optionally the catalyst has a ratio of cobalt tomanganese ranging from about 3:1 to about 1:2 by weight on an elementalmetal basis.
 6. The mixture of claim 1 further comprising an oxidationproduct comprising at least one of an aromatic aldehyde, an aromaticalcohol, an aromatic ketone, and an aromatic carboxylic acid.
 7. Themixture of claim 1 wherein the bromine source is at least one of HBr,NaBr, KBr, NH4Br, benzylbromide, mono-bromoacetic acid, di-bromoaceticacid, bromoacetyl bromide, tetrabromoethane, and ethylene di-bromide.